1. Field of the Invention
The invention relates to a ruthenium or rhodium catalyst for hydrogenating methylenedianiline (MDA) to bis-para-aminocyclohexylmethane (PACM), in which the catalytically active metal is applied to a support made of a special transition alumina.
2. Discussion of the Background
The hydrogenation of methylenedianiline was developed around 1947 and applied on an industrial scale around 1965. Most disclosures describe processes which are carried out in suspension with the addition of a substantially unreactive solvent. The hydrogenation product PACM has been gradually used in more and more applications in which the isomer ratios are very important.
U.S. Pat. No. 2,511,028 and U.S. Pat. No. 2,606,925 describe, in general terms, the preparation of PACM by hydrogenation of MDA over a ruthenium catalyst. U.S. Pat. No. 2,606,928 shows the preparation of PACM having an increased cis-content; the catalyst is ruthenium with addition of alkali on activated carbon. A further development is described in U.S. Pat. No. 3,697,449, the alkali-moderated catalyst being used on supports of aluminum oxide, barium sulfate or kieselguhr in pulverulent form; according to U.S. Pat. No. 3,636,108 the alkali-moderated catalyst is additionally admixed with alkali metal amide or methoxide.
U.S. Pat. No. 3,636,108 and U.S. Pat. No. 3,644,522 describe how the trans/trans isomer content can be shifted during the hydrogenation or subsequently. The catalyst used is ruthenium, if desired modified with alkali, on a weakly alkaline or amphoteric support such as potassium carbonate or alkaline earth metal oxide. Further properties of ruthenium catalysts, of which mention may be made, are disclosed in U.S. Pat. No. 4,448,995 and U.S. Pat. No. 4,394,298 and also DE-A 40 28 270, DE-A 27 45 172 and DE-A 25 20 848.
According to U.S. Pat. No. 3,591,635, U.S. Pat. No. 3,856,862 and EP-A 0 662 212 and EP-A 0 392 435, rhodium catalysts are used for the hydrogenation.
Finally, European Patent 0 324 190 describes a process for hydrogenating MDA in a fixed bed; the addition of solvent is merely optional. Use of a ruthenium catalyst (0.1-5% of ruthenium) on alumina with a relatively deeply impregnated shell (at least 50 .mu.m), a BET surface area of from 70 to 280 m.sup.2 /g and an average pore diameter of from 1.0 to 32.0 nm makes it possible to achieve high activity and a low proportion of trans/trans isomers. The process is preferably carried out in a solvent-free melt.
From the abovementioned literature citations, it is evident that the hydrogenation of substituted anilines, in particular MDA, to products having a high proportion of cis or cis/cis isomer, presents a significant problem, particularly with regard to finding a catalyst which promotes this selectivity. This applies particularly to carrying out the reaction in a fixed bed and could be the reason for the low success rate in this area.
Furthermore, it is apparent that there is a preconception that the isomer distribution of the product can only be affected by altering the reaction conditions, which must be particularly accurately adhered to, by doping the support with alkali metal or alkaline earth metal compounds or by the addition of ammonia during the reaction. The measures of alkalization and addition of ammonia are also said to effect a suppression of the deamination and the isomerization of amino groups. However, lowering the reaction temperature has technical limits because of diminished catalyst activity. In addition, such a doping with alkalis obviously makes it necessary, because of their good solubility, to apply the metallic active components of the catalyst in a relatively thick layer. However, the inward and outward transport of the reactants by diffusion is thereby made much more difficult, which, despite the relatively slow reaction, leads to the reaction rate and selectivity being controlled by the rate of material transport and not by the intrinsic performance of the active component. In addition, the channels of such deeply impregnated catalysts are all the more easily blocked by highly viscous materials such as high boilers, oligomers, carbon deposits, and the like as the average pore radius and pore volume become smaller. Additionally, a large BET surface area and a large average pore radius are required for optimal distribution and dispersion of the metal particles.
The process according to European Patent 0 324 190 already shows good activity values and a relatively small proportion of trans/trans isomer. However, the mechanical stability of the alumina supports used therein requires a high pretreatment temperature, through which the surface of these materials is relatively quickly deactivated.
Furthermore, studies of reaction kinetics have shown that the isomer distribution is not only a result of the acidity of the catalyst, but also of the residence time, and an increase in the latter shifts the distribution towards higher trans/trans contents, i.e. in the undesired direction, because the catalyst isomerizes more strongly. With increasing covering of the inner pores of a thick-shell-impregnated catalyst support, the diffusion coefficients of starting materials such as MDA and products such as PACM fall relatively quickly, so that after a relatively short catalyst operating time, not only does deactivation occur, but also the residence time increases in the catalyst interior and with it a shifting of the selectivity into unfavorable ranges. Since these factors cannot be compensated by an increase in temperature, because this would shift the selectivity further into the undesired range, the catalyst for such processes generally has to be replaced after a relatively short time. A need therefore continues to exist for a more effective catalyst for the hydrogenation of methylenedianiline.